You're a little off on the cam terminology. "Flat tappet" and "hydraulic" describe two different things. Tappet type can be either flat or roller, and lifter operation can be either hydraulic or solid. You confused these terms in your post, but here is a quick and dirty breakdown:

Flat tappet vs. roller: Higher valve lift can be had from a roller cam profile than a flat tappet given the same duration. Generally the aim, especially on a street-driven car, is to maximize lift as much as possible without using a duration and overlap that puts the engine's power band too high up in the RPM range where it will be a bear to drive on the street. Roller has an edge here (along with reduced friction and longer life) but it can be a lot more expensive.

Hydraulic vs. Solid lifters: Hydraulic lifters use oil to automatically adjust the "size" of a lifter to make up for lash in the valvetrain caused by changing conditions within the engine, such as temperature and wear. Solid lifters are just that; they do not adjust automatically and so must be adjusted by you, so to speak. You are right in that solid lifters are more suited to high-RPM operation than hydraulic, however in a regularly street-driven car, your most-often used RPM range will not really necessitate solid lifters.

To run a roller cam setup, you either need to buy a block such as those you described, which are cast with the appropriate bosses in the lifter valley for the "spider", or you can buy retrofit roller lifters (very expensive) or machine an older block to accept the setup (not really worth it in my opinion).

Two-bolt blocks should be OK up to about 500 HP or so as long as you don't flog it at constantly high RPM. Factory rods should be fine, and factory cranks might be as well but you'll want to make sure it is in good, usable condition (no cracks or signs of fatigue).

The Dart IE heads should be fine for your power goals and as far as the runner size is concerned, just make sure that it matches your cam specs and the intended use of the car (bigger runners for high-RPM horsepower, smaller runners for low-RPM street torque).